1. Field of the Invention
The invention is concerned with the manufacture of transmission lines for visible and near-visible electromagnetic radiation.
2. Description of the Prior Art
The emerging technology of fiber optics promises to be of commercial significance in broad bandwidth communications, a field which is currently dominated by microwave transmission. In particular, where cable space is at a premium, a significant increase in transmission capacity can potentially be realized by replacing presently used microwave carriers with hair-thin fibers carrying modulated visible or near-visible electro-magnetic radiation. In the following, such radiation (more specifically, electromagnetic radiation of a wavelength in the range of from 0.5 to 2 micrometers) will be designated as "light." For the purpose of transmitting light, glass fibers of a clad structure have been proposed. In such fibers, light guiding is effected by a decrease in refractive index from the center of the fiber outwards towards its periphery. Typically, the fiber is of a structure which comprises a core portion throughout which the refractive index is essentially constant and a cladding of a lower constant refractive index. Alternatively, a structure of graded index has been proposed in which the refractive index decreases gradually in directions away from the center line. In this latter structure the grading can be chosen so as to minimize mode dispersion when the fiber is intended for multimode use, a parabolic grading being particularly advantageous for this purpose.
A number of manufacturing processes have been proposed for the fabrication of optical fibers. For example, according to the method disclosed in U.S. Pat. No. 3,775,075 issued to Keck et al. on Nov. 27, 1973, a cylindrical fused silica rod doped, to increase its refractive index, with a material such as titanium dioxide is coated with a layer of silica soot. The resulting assembly is heated, first to sinter the soot into a glass, and then to draw the assembly into a thin fiber. Conversely, U.S. Pat. No. 3,778,132 issued to Pinnow et al. on Dec. 11, 1973, discloses an optical fiber with a core of essentially pure silica and a cladding of silica doped with a sufficient amount of boron trioxide to achieve a refractive index of at least 0.1 percent lower in the cladding as compared with that of the core. This silica-core, borosilica-clad fiber has a number of desirable features, among which are purity and homogeneity of the core material, ease of joining of fibers by heat fusing the cladding without disturbing the core, and hermetically sealing the fiber through pyrex. The latter feature is due to a close match in thermal expansion coefficients between borosilicate and pyrex and is of importance in connecting optical fibers to pyrex-encapsulated optical devices.